# std::indirect_binary_predicate

From cppreference.com
< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class F, class I1, class I2 > concept indirect_binary_predicate = std::indirectly_readable<I1> && std::indirectly_readable<I2> && std::copy_constructible<F> && std::predicate<F&, /*indirect-value-t*/<I1>, /*indirect-value-t*/<I2>> && std::predicate<F&, /*indirect-value-t*/<I1>, std::iter_reference_t<I2>> && std::predicate<F&, std::iter_reference_t<I1>, /*indirect-value-t*/<I2>> && std::predicate<F&, std::iter_reference_t<I1>, std::iter_reference_t<I2>>; |  | (since C++20)

The concept `indirect_binary_predicate` specifies requirements for algorithms that call binary predicates as their arguments. The key difference between this concept and std::predicate is that it is applied to the types that `I1` and `I2` references, rather than `I1` and `I2` themselves.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2609R3 | C++20 | some requirements were defined in terms of std::iter_value_t<I>& which mishandled projections resulting in incompatibility with predicate F& | defined in terms of /*indirect-value-t*/<I> to correctly handle such projections
P2997R1 | C++20 | `indirect_binary_predicate` required F& to satisfy `predicate` with std::iter_common_reference_t<I> | does not require

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/iterator/indirect_binary_predicate&oldid=175479"